Fears that Beijing's awful air would hinder performances might have been dispelled by the way the swimmers pulverized world records. But the real test of the city's pollution controls will come when endurance athletes race outside in Beijing's hot, humid and polluted air. There have been casualties already: more than a third of the cyclists competing in the 152-mile (245 km) men's road race Aug. 9 dropped out, in part because of conditions so stifling that one rider compared it with racing at 10,000 ft. (about 3,000 m)--on a course that topped out at 1,083 ft. (330 m). Olympians in Beijing are breathing a soup of pollutants--including ultrafine particulates, carbon monoxide, sulfur oxides and ozone, each of which might slow them down. A recent study shows why. Kenneth Rundell of Marywood University in Scranton, Pa., had 15 college hockey players bike flat out for 6 min., breathing first low-, then high-particulate air. By their second ride in polluted air, the subjects' performance had decreased an average of 5.5%. For a marathoner, that's a loss of up to 7 min.--enough to put a world record well out of reach.